Permanent magnet direct current (DC) motors are in wide use in applications that require moderate torque within a small volume. The torque is generated by the interaction of two magnetic fields: a fixed magnitude one generated by a set of permanent magnets, and a variable amplitude one generated by a set of electromagnet coil windings. A high magnetic permeability material, such as electrical iron, is used to direct the magnetic flux to produce the largest possible interaction between the two fields. In brushless permanent magnet DC motors, the coil windings form part of the stator and remain stationary, and the permanent magnets form part of the rotor and rotate. The permanent magnets are arranged with alternating polarity, and as the rotor spins, the current in the coil windings is switched to produce the desired torque.
Brushless permanent magnet DC motors are used in spindle motors in small magnetic recording disk drives. The spindle motor rotates a hub that supports the disks. These are low-voltage motors; typically 12 V in 3.5 in. form factor disk drives and 5 V in 2.5 in. form factor disk drives. The permanent magnet is typically a ring magnet that generates a magnetic field in a radial direction across a radial gap. The coils are wound around radially oriented stator teeth and produce radially directed magnetic fields across the radial gap. An example of a disk drive with a radial-gap spindle motor is described in U.S. Pat. No. 5,352,947. The advantage of a radial-gap spindle motor is that it produces only small radial forces and no axial forces on the motor bearing assembly.
The torque generated in such motors is dependent on the number of amp-turns that each coil can carry. High torque is important to start the disk drive, especially in those disk drives that start and stop with the recording heads in contact with the disk surfaces since there is a static friction force that must be overcome at start-up. Therefore for high torque it is important that the coils have a large number of turns but that the overall electrical resistance of the coils be kept low. This is especially important for the low voltage disk drive motors. The primary disadvantages of such motors are the limited volume allowed between adjacent teeth and the requirement that the coil windings extend axially at least as high as the axial height of the stator teeth. In such a motor the coil wire length needs to be very long, which causes the motor to have high electrical resistance and therefore high losses and high self-heating. Also, because the coils are wound around the stator teeth, there must be some angular spacing of the stator teeth near the working gap to allow for insertion of a coil winding guide to fit between adjacent stator teeth. This increased spacing of adjacent stator teeth increases the cogging torque of the motor.
Co-pending application Ser. No. 08/400,645 filed Mar. 8, 1995 is directed to a radial-gap spindle motor that uses flat coils to generate axial fields and a ring flux guide to direct the flux circumferentially. In that invention, the flat coils, the stator teeth and the radial gap are generally in the same plane, and the stator teeth are located outside the motor hub.
What is needed is a disk drive radial-gap spindle motor with a large volume allowed for low resistance coils wound in a maximum number of turns, but with a small or minimal spacing between adjacent stator teeth to minimize the cogging torque of the motor.